The invention relates generally to plasma applicators.
Plasma-based, excitation sources or plasma applicators must often be able to handle high input powers and withstand high temperatures combined with a highly reactive chemical environment. For example, in a common application of a plasma applicator, NF.sub.3 gas flows into the applicator and is broken down by the plasma. The resulting activated species flows out of the applicator and into the semiconductor processing equipment where it is used for in-situ chamber cleaning, etching, photo resist stripping, or any of a number of other tasks. As an example of using the reactive species for in-situ chamber cleaning refer to U.S. Ser. No. 08/278,605, entitled "A Deposition Chamber Cleaning Technique Using a Remote Excitation Source" filed on Jul. 21, 1994, now abandoned and incorporated herein by reference.
The extremely hostile environment to which such equipment is exposed can disable the plasma applicator very quickly. For example, some applicators that are commercially available use quartz tubes to contain the activated species. In those systems, the fluorine that is produced etches the tube quite quickly. Moreover, at high power levels (e.g. above 1 kW) the quartz will tend to break down. Thus, after using the applicator only a few times or for a sustained period of operation, the wall of the tube will become so thin that it will collapse under continued exposure to the high temperatures and the vacuums that are used in such systems. Thus, very early in the life of the tube, it must be discarded and replaced with a new tube. Both the inconvenience and cost of having to repeatedly replace the quartz tube can be quite high.
Some existing plasma applicators use ceramic tubes in place of the quartz tubes. Ceramic tubes are capable of holding up better than the quartz tubes in the chemically corrosive environments often encountered. But ceramic tubes are not a panacea. They typically have a relatively high thermal expansion coefficient as compared to quartz and other materials. Thus, repeated cycling between room temperature and the high processing temperatures that commonly occur in these systems produces large stresses within the ceramic tubes. These stresses eventually result in the tubes cracking and failing.
Some microwave plasma applicators have been developed which use two concentric tubes, namely, an outer tube and an inner tube both of which are made of materials that are transparent to the microwave radiation, e.g. quartz and sapphire. The inner tube contains the plasma and thus is the tube that is exposed to the high temperature and corrosive conditions. To cool the inner tube, water is flowed through the annular region between the two tubes. Such a system is described in U.S. patent application Ser. No. 08/387,603, filed Feb. 13, 1995, now abandoned, and incorporated herein by reference. Since water absorbs microwaves, there is a severe restriction on how thick the annular region can be made. If it is too thick, the microwaves will be seriously attenuated and it will be difficult or even impossible to initiate and sustain a plasma within the inner tube. On the other hand, if this region is too thin, cooling efficiency will be seriously compromised.
In spite of these advances in plasma applicator design, the high thermal stresses produced by thermal expansion of the plasma tube still results in cracked tubes. In addition, the plasma tubes are not the only components that fail. Seals and O-rings which aid in maintaining a vacuum within the plasma tube and which aid in sealing the coolant system also rapidly deteriorate and eventually fail when exposed to the high temperatures and other hostile conditions produced in such systems. Moreover, as the power levels are increased beyond the 1 kW levels, the failure problems in currently available plasma applicators become even more severe and the frequency of failures rises.